An automobile engine generally includes a transmission device, which utilizes an endless, traveling, flexible, transmission medium, such as a roller chain, silent chain, toothed belt, or the like, to transmit rotation from the engine crankshaft to one or more valve-operating camshafts. In such a transmission device, sliding contact guides are used to guide the chain. Typically, a fixed guide is disposed in sliding engagement with the tension side of the chain, i.e., the side moving from a driven sprocket to the driving sprocket, and a movable guide is disposed in sliding engagement with the slack side of the chain, i.e., the side moving from the driving sprocket to a driven sprocket. Both guides control the path of the chain to prevent vibration, both in the plane of movement of the chain and in directions transverse to the plane of movement of the chain. The movable guide additionally cooperates with a tensioner to maintain appropriate tension in the chain. The movable guide is typically pivoted on a mounting pin extending from the engine block, and the fixed guide is typically attached to the engine block by mounting bolts. A conventional transmission device of the kind described above is depicted and explained in detail in U.S. Pat. No. 6,086,498, granted Jul. 11, 2000, the entire disclosure of which is incorporated here by reference.
Each guide usually comprises a resin shoe that is fixed to a metal guide base. The resin shoe has a guide surface on which the transmission medium slides, and a back surface which abuts the metal guide base. FIG. 4 shows the back of a typical shoe 100 of the kind used in a conventional fixed guide. The back 120 of the guide is provided with a central, longitudinal rib 140, and a plurality of perpendicular ribs 142.
In the conventional resin shoe of the kind depicted in FIG. 4, it is difficult to remove heat from the central portion of a chain-contacting surface during molding. Consequently, deformation is liable to occur as a result of thermal shrinkage and warping following molding. Another problem encountered in the molding of the conventional shoes is that the size of the finished product is difficult to control, and consequently significant size variations have been observed.
The principal object of the invention is to avoid the above-mentioned problems encountered in the manufacture and use of conventional sliding contact guides, to provide a sliding contact guide having improved strength without increasing its weight, to avoid deformation of the guide during and following molding, and to minimize size variations in the finished products.
The sliding contact guide in accordance with the invention comprises a metal guide base, and an elongated resin shoe fixed to the guide base. The shoe has a front guide surface, facing in a first direction, for sliding contact with a flexible transmission medium traveling along the direction of elongation of the shoe. The back of the shoe faces opposite to the first direction, and is disposed in abutting relationship with the guide base. The shoe also has a pair of opposite sides, and both its front guide surface and its back are situated between the sides. The back of the shoe comprises a pair of elongated first ribs extending longitudinally along the shoe in spaced relationship to each other, and a lattice composed of elongated second ribs disposed between the first ribs. The widths of the first ribs, measured in directions perpendicular to the direction in which the back of the shoe faces, are greater than the widths of the second ribs, also measured in directions perpendicular to the direction in which the back of the shoe faces.
The longitudinal first ribs preferably form parts of the pair of opposite sides of the shoe, and both the first ribs and the second ribs are preferably in contact with a front surface of the base.
The term “lattice,” as used herein, refers to a configuration of ribs in which a first rib or set of generally parallel ribs is in intersecting relationship with a second rib or set of generally parallel ribs, where corresponding parts of the ribs are disposed in a common surface such as a plane or a gradually curved surface, which may be defined by a simple, two-dimensional curve, or which may be a compound curve. The respective sets of ribs of the lattice can be disposed longitudinally and transverse to the longitudinal direction of the shoe, or all the ribs can be oblique relative to the longitudinal direction of the shoe. The ribs of the two sets need not intersect one another perpendicularly.
Any of a wide variety of materials may be used to form the resin shoe. However, since the sliding contact surface of the shoe is in engagement with a moving chain or other traveling power transmission medium, the resin shoe is preferably composed of an engineering plastics such as polyamide base resins and the like, which exhibit excellent wear resistance and lubricity. Preferably materials such as nylon 6, nylon 66, aromatic group nylons, and the like are used. Fiber-reinforced plastics may be also be used, either by themselves or in combination with other materials, depending on factors such as strength requirements.
Provided that the guide has adequate heat resistance and strength, any of a wide variety of materials may be used for the metal guide base. However, iron-base metal such as cast iron, stainless steel and the like, and non-ferrous metals and metal alloys, comprising aluminum, magnesium, titanium or the like as the main component, are preferred.
The spaced, longitudinal ribs, which preferably extend along the sides of the shoe, allow uniform cooling during thermal shrinkage. The ribs of the lattice formed between the longitudinal ribs ensure that the guide has adequate strength to serve as a sliding contact guide for a travelling transmission medium. Moreover, since the widths of the longitudinal ribs are greater than the widths of the lattice ribs, deformation of the shoe due to thermal shrinkage following molding is suppressed.
Suppression of thermal shrinkage after molding affords a greater degree of freedom in the design of the shoe, and reduces the size variation of the finished product so that consistent high quality can be achieved. Suppression of thermal shrinkage of the shoe also reduces deformation of the sliding contact surface, so that a chain or other traveling transmission medium can run over the shoe in a stable manner with a minimum of wear. As a result, the service life of the shoe is extended.